In thermal power systems for generating power by combusting fossil fuels such as coal and heavy fuel, a large amount of CO2 is discharged. In recent years, the greenhouse effect by CO2 is pointed out as one of the causes of the global warming. It is necessary, therefore, to take prompt measures to protect the global environment on an international basis. Under these circumstances, there is a trend toward further increase in demand for CO2 discharge control.
In the thermal power systems, there is installed an exhaust gas treatment system including a CO2 recovery unit placed in a flow passage on the downstream side of a boiler. The CO2 recovery unit includes a CO2 absorption column and a CO2 regeneration column as described in Patent Literature 1 and Patent Literature 2. In the CO2 absorption column, combustion exhaust gas from the boiler is brought into contact with an amine CO2 absorbing solution to remove CO2 in the combustion exhaust gas. The absorbing solution which absorbed CO2 is conveyed to the regeneration column. In the regeneration column, the absorbing solution which absorbed CO2 is heated and CO2 is released from the absorbing solution. As a result, the absorbing solution is regenerated. The absorbing solution regenerated in the regeneration column is circulated to the CO2 absorption column and is reused therein. The CO2 recovered from the combustion exhaust gas by the CO2 recovery unit is stored without being released to the air or is used for Enhanced Oil Recovery (EOR) and the like.
Since the CO2 recovery unit is placed as an additional structure of a combustion facility, it is also necessary to minimize its operating cost. In the step of regenerating the absorbing solution in particular, a large amount of thermal energy is consumed. Therefore, the regenerating process needs to be energy-saving as much as possible.
In the exhaust gas treatment system in Patent Literature 1, a part of an absorbing solution which was extracted through a bottom portion of the CO2 absorption column and supplied to the CO2 regeneration column is preheated through heat exchange with vapor of a reboiler of the CO2 regeneration column, before heat exchange is performed between the absorbing solution and a regenerated absorbing solution which was extracted through a bottom portion of the regeneration column and supplied to the CO2 absorption column. With such a structure, sensible heat of the absorbing solution in the regeneration column increases, so that the vapor amount fed to a CO2 removal facility is reduced.
In the thermal power system in Patent Literature 2, the amount of heat recovered by cooling the exhaust gas discharged from the boiler is used to heat an absorbing solution which absorbed CO2 and which is supplied from the CO2 absorption column to the regeneration column. Moreover, condensate from a condenser which returns water vapor sent from a turbine to water is used to cool washing water for use in collection of the absorbing solution in the CO2 absorption column, while the condensate is also used to cool CO2 separated in the regeneration column.